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All rapid responses

Rapid responses are electronic letters to the editor. They enable our users to debate issues raised in articles published on thebmj.com. Although a selection of rapid responses will be included online and in print as readers' letters, their first appearance online means that they are published articles. If you need the url (web address) of an individual response, perhaps for citation purposes, simply click on the response headline and copy the url from the browser window. Letters are indexed in PubMed.

We read with great interest the prospective cohort study undertaken by Smith-Bindman and colleagues that quantitates the international variation in radiation dose for computed tomography (CT) examinations (1). As discussed in a related editorial (2), this study is the first to identify the selection of protocol parameters as the most significant cause of dose variation over and above patient characteristics. It highlights up to a fourfold range in mean effective dose for abdominal and chest CT examinations and emphasises the importance of keeping doses in diagnostics as low as reasonably practicable for their intended use.

With respect to optimising patient centred care, we were pleased to see active patient and public participation in guiding the direction of this project. In a separate study (3) undertaken by The Royal College of Radiologists and The Patients Association that aimed to capture the reality of patients’ experiences of diagnostic imaging, an unacceptable 23% of patients reported that they did not receive any explanation for having their X-ray or scan. Historically, shared decision making through fostering of patient-clinician relationships has largely been focused on treatment decisions. We feel that it would be valuable to extend this collaborative approach, facilitated by respectful and informed discussion, to incorporate the use of diagnostic imaging.

Given that all medical exposure to ionising radiation including from CT examinations is associated with a hypothetical increased cancer risk, it is essential to minimise exposure from medical imaging and ensure that the benefits outweigh the risks. However, this is not always straightforward as current data on radiation exposure and cancer risk is based on data from survivors of atomic bombs, nuclear accidents and the early use of x-rays. In addition to quantitating the level of exposure, a number of other factors to be taken into consideration (4). The estimated risk is context dependent to the patient age, the disease and the prognosis. For example, by estimating the number of additional cases of cancer attributable to a single dose of 100mSv for different age groups and taking into account radiation exposure to medical imaging studies (5) (6), the fatal cancer risk from five CT chest, abdomen and pelvis scans (totally 100mSv effective dose) to a healthy forty year old is about 1 in 200 (0.5%). In comparison, if the same number of scans (with the same radiation dose delivered) were to be undertaken on a healthy ten year old child, the fatal cancer risk increases to about 1 in 65 (1.5%). Conversely, if the same CT Scans were carried out on a patient with an expected prognosis/survival of less than 2-3 years, the fatal cancer risks fall to a negligible (less than 1 in 100,000 – 0.001%) level. When it comes to deciding on the appropriate diagnostic examination, the radiation dose, estimated risk as well as the respective diagnostic accuracy of different imaging modalities need to be considered (7).

In their manuscript, Berger and colleagues discuss the nature of the patient’s concerns, in particular how they manage the uncertainties of diagnostic imaging (8). The authors offer pragmatic guidance on how to approach informed conversations with patients based on the level of uncertainty and risk of diagnostic scans; clinicians should seek to obtain consent for all patients before proceeding with a scan but in depth conversations should be reserved for situations where there is limited evidence to accurately weigh the risks and benefits of diagnostic evaluation.

Our hope in writing this letter is not to suggest that clinicians already working in extremely busy clinics should be inappropriately over-burdened. Instead, we wish to highlight the value of considered patient-clinician conversations relating to diagnostic testing especially in situations where the benefits and risks of diagnostic evaluation are difficult to quantify.

The article by Smith-Bindman et al. (1) raises concerns about the variation in the radiation doses of CT examinations across institutions. It states that ionizing radiation is a known carcinogen and that CT radiation is associated with increased cancer incidence. Reducing unnecessary variation and optimizing protocols are advocated to minimize the dose from medical imaging.

Dr. Mohan Doss (2) challenged the claim that low doses of radiation cause cancer by pointing out that the publications that were quoted in support of this idea are based on the unsupported linear no-threshold dose-response model of cancer risk assessment and flawed studies of radiation workers. He argued that dose reduction would not benefit patients, i.e., no decrease in cancer risk, and could harm those who might be diagnosed incorrectly due to low-quality images having an inadequate spatial resolution.

Radiation biology has made great strides over the past 30 years; we have a much clearer picture of how low doses of radiation affect organisms (3). In addition, there is recently-identified human evidence (4,5) of a high dose threshold, 1.1 Gy (68% CI: 0.60-2.1), for radiation-induced leukemia—a cancer of hemopoietic stem cells, which have a high radiation sensitivity compared with the radiation sensitivities of stem cells in other organs. This threshold significantly exceeds the dose range of CT scans, 20 to 100 mGy (6), and gives an indication of the safety of medical diagnostic imaging using low doses of ionizing radiation.

The prospective cohort study by Rebecca Smith-Bindman and colleagues is of immense interest and has great clinical significance. Variation in radiation doses for computed tomography (CT) examinations is not limited to seven countries, but it is a major problem all over the world.

Who is responsible for this variation in radiation doses for CT examinations? The study revealed that it is the “Radiology Teams & Medical staff”, not the patients, institution, or machine characteristics.

During a CT examination, the radiation dose transmitted to the patient can be very high. CT radiation is a known carcinogen and associated with an increased risk of cancers. Hence it is important to minimize the radiation exposure to patients without compromising image quality and diagnosis.

Multiple slices CT (MSCT) scanners with a range of 2, 4, 8, 16, 32, 64 and 256 slices are used in clinical practice. It is pertinent to mention here that higher slice CT scanners like 32, 64 and 256 slice provide larger organ coverage & better image quality in less scan time, but also deliver higher radiation exposure to patients.

Studies have revealed that CT radiation doses can be reduced significantly without reducing image quality and diagnostic accuracy. If the image quality produced by the 16 or 32 slice CT scanner is good & acceptable for diagnosis, then do not use higher slice CT scanners [1-4].

Hence it becomes the responsibility of the Radiologist and their team regarding the selection & judicious use of MSCT scanners and optimizing radiation doses to avoid unnecessary radiation exposure to patients and in the work place.

This letter is regarding the article by Smith-Bindman et al. (1) which aimed at determining the patient, institution, and machine characteristics that contribute to variation in radiation doses used for computed tomography (CT). In their prospective cohort study, the authors reported a substantial variation in radiation dose for computed tomography (CT) examinations across patients, institutions, and countries. The carcinogenicity of exposure to low doses of ionizing radiation is addressed by Smith-Bindman et al. by providing evidence that support the so-called linear no-threshold (LNT) model.

Although this paper addresses a challenging issue, relying on the LNT hypothesis to evaluate the radiation risk at low doses and dose rates can be misleading. While the LNT hypothesis assumes detrimental effects arise at the cellular level and are linked to DNA damage, it does not specifically address subsequent DNA repair mechanisms. In particular, some evidence shows that the most effective repair mechanisms can be observed at low doses. Base excision repair (BER), nucleotide excision repair (NER), and mismatch repair are the three fundamental mechanisms associated with DNA repair (2). Substantial evidence shows that radiation-induced DNA damage is significantly less severe than the spontaneous DNA damages that occur from a wide variety of other factors ranging from chemicals, heat, metabolic transients and natural background radiation. It should be noted that less than 0.1% of DNA base changes create a permanent mutation (3). Most are efficiently eliminated by the DNA repair mechanisms. Given this consideration, the frequency of mutations created by exposure to low dose radiation is significantly less than natural mutations. DNA repair and other human body natural defense mechanisms, including the immune system, provide a robust system to protect the body from a wide range of detrimental agents. These repair mechanisms suggest there are flaws in the conclusions of Smith-Bindman et al.

Moreover, reviewing the epidemiological studies shows lack of consistent evidence to support the cancer concerns regarding the exposure to low levels of diagnostic X-rays(4). While, the ICRP-recommended annual effective dose limit for radiation workers is 20 mSv, effective dose in some high background radiation areas is up to 260 mSv/y (13 times higher than the limit for occupational exposures)(5, 6). In spite of this, exposure to these elevated levels of natural radiation has not increased the risk of either acute effects or cancer “Background radiation has never been shown to unequivocally cause acute or latent disease, such as cancer (Hall and Ciaccia 2005)” (7). Moreover, there are reports showing reduced cancer rates in the residents of areas with elevated levels of natural radiation “Reduced cancer occurrence was reported since decades ago for HBRAs (Frigerio et al. 1973, Cohen 1995, Aliyu and Ramli 2015, Mortazavi et al. 2005, Nair et al. 2009, Sun et al. 2000)”(7). Therefore, if exposure of humans to radiation doses of a few hundred mSv per year is detrimental to their health causing increased risk of cancer, it should be evident in these people. In spite of this, nearly all residents still live in their paternal dwellings and there are not consistent reports on any detrimental effects(8, 9).

I am responding to the article by Smith-Bindman et al. (1) which reported a substantial variation in radiation dose for computed tomography (CT) examinations across patients, institutions, and countries. The article recommended that radiation dose from CT scans should be optimized to reduce the variation, in consideration of the cancer risk due to the CT scans. Many professional organizations (2-4) have also recommended optimization of CT radiation dose based on similar arguments. I am concerned that this article presents a misleading picture of the cancer risk from the low levels of radiation involved in CT scans.

Smith-Bindman et al. quoted several publications in claiming the carcinogenicity of low levels of radiation. One publication they quoted (5) utilized the linear no-threshold (LNT) model to estimate the cancer risk due to low radiation doses. However, as explained in a recent publication (6), the LNT model is not supported by epidemiologic studies, though such claims have been made by many advisory body reports over the years, including a recent report by NCRP (7). On the other hand, many studies (6) support the opposite concept known as radiation hormesis (8) according to which cancer risk reduces with low radiation exposures. Another publication they quoted is the BEIR VII report (9). However, the main evidence quoted by the report, the atomic bomb survivor cancer mortality data, no longer supports the LNT model following the 2012 update (10), due to the significant curvature in the dose-response relationship, and the data are consistent with radiation hormesis (11). The 15-country study of radiation workers (12), which was quoted by the article and which was also used by the BEIR VII report to support cancer risk of low radiation doses, does not show significantly increased cancer risk in the radiation workers when the faulty Canadian data are excluded (13). The authors also quoted the INWORKS study on nuclear industry workers in France, UK, and USA (14). One major flaw of this study is that it did not correct for the likely major confounding of the data due to the much higher smoking rates in the radiation workers during the early period of the study when most of the higher doses among the radiation workers would have occurred (6). The conclusions of the two studies on cancer risk following childhood CT scans (15,16) quoted by the authors have already been discredited (17). Another publication (18) they quoted refers to these two studies. Two of the other references quoted by the authors to claim the carcinogenicity of low-dose radiation (19,20) refer to the BEIR VII report whose reliability is questionable, as mentioned earlier. Thus, even though the authors quoted a large number of publications to support their concerns regarding the radiation dose from CT scans, on scrutiny, these publications do not provide valid evidence for the carcinogenicity of low levels of radiation.

In view of the above discussion, the carcinogenic concerns raised by the authors regarding the low radiation doses from CT scans are not valid. Hence, the dose reduction efforts they recommend would not benefit patients by reducing their cancer risk. On the other hand, reducing CT doses can result in lower quality images and reduce the confidence in diagnoses (21). Though the authors claimed that a factor of 2 or more in dose reduction could be achieved without affecting diagnostic accuracy by quoting a publication from 2007 (22), more recent analyses indicate that a factor of two in dose reduction can adversely affect spatial resolution (23) and diagnostic accuracy (24). A review of pediatric CT scans submitted for establishing diagnostic reference levels indicates that 1 in 20 pediatric abdominal CT scans were inadequate for diagnostic purposes due to excessive radiation dose reduction efforts (25). These studies indicate that there is a potential to cause serious harm to patients due to the CT dose reduction/optimization efforts, and the dose reduction would provide no benefit to patients.

In summary, the recommendation to optimize radiation dose from CT scans is based on faulty publications that have claimed increased cancers due to low radiation exposures. CT dose reduction has the potential to harm patients due to poorer quality scans and the consequential misdiagnoses without any benefit of reducing cancer risk. Therefore, the authors (as well professional organizations that recommend CT dose optimization) should rescind such recommendations.